In subsea hydrocarbon drilling operations, a tubing hanger is attached to the topmost tubing joint in the wellhead to support the tubing string. The tubing hanger is typically located in a tubing hanger housing, with both components incorporating a sealing system to ensure that the tubing conduit and annulus are hydraulically isolated.
Seals are used between the tubing hanger and the tubing hanger housing to seal off wellbore pressure. The use of multiple seals forms a fixed-volume annular region between the tubing hanger, the tubing hanger housing, and the seals and does not allow pressure to be released from this region. In some instances, a fluid such as seawater may occupy this annular region. During hydrocarbon production, the temperature of the fluid in the annular region may increase, for example as a result of the increased temperature of the hydrocarbons flowing through the tubing string supported by the tubing hanger. Increasing the temperature of a fluid in a fixed volume can greatly increase the pressure of the fluid. This increase in pressure may challenge the integrity of the annular seals, tubing hanger, or tubing hanger housing.